1. Field of the Invention
The invention relates to seismic exploration, and more particularly to deterministic methods of analyzing and processing seismic data using a source pulse waveform.
2. Description of the Related Art
Generally, the oil and gas industry developed seismic exploration to determine the location and character of subterranean geological formations. Seismic exploration is not limited merely to land, however, but is also widely applied to find mineral deposits in submarine geological formations. Using the data acquired from seismic exploration, an analyst may construct a model, of the submarine geology and determine potential mineral deposit locations.
To explore submarine geological formations, an exploration vessel tows an acoustic source through the water. A variety of acoustic sources are available, including air guns, water guns, and marine vibrators. The acoustic source emits acoustic pulses, or pressure pulses, at selected locations or intervals. The pressure pulse propagates through the water to the marine floor. When the pressure pulse strikes the water/earth interface at the bottom, part of the energy of the pulse is transmitted into the geological formation and part is reflected upward into the water. As the energy from the transmitted part of the pulse propagates downward through the geological formation, the pulse encounters boundaries where the physical characteristics of the geological formation changes due to sedimentary layering or other geological phenomena. Because of the acoustic impedance of the boundary, part of the energy of the propagated pulse is reflected upwards from each boundary towards the surface. Sensors on the surface detect the energy of the reflected pulses. The sensors are commonly drawn by the vessel towing the acoustic source, or may be drawn by separate vessels. The sensors record the reflected pulses, and the various characteristics of the data are later analyzed to determine the attributes of the submarine formation.
Geophysicists and geologists analyze seismic exploration data from the reflected pulses with a variety of methods. The practicality and reliability of the techniques vary widely, and each involves certain advantages and disadvantages. Consequently, the data analyst's preferences and experience typically determine which data analysis technique is applied. Most data analysis techniques, however, rely upon a convolutional model of the seismogram. Generally, the reflected waves detected by the exploration vessel may be expressed as a convolution function, dependent upon a time function of the source pulse convolved with an impulse response function characteristic of the structure of the geological formation. In the convolution model, the source pulse function and the geologically characteristic function are independent and separate. Thus, the detected waveform may be deconvolved to separate the source pulse function from the geologically characteristic function. Subsequent analysis of the geologically characteristic function then indicates the features and characteristics of the submarine formation.
Because the deconvolution process requires separation of the source pulse function to establish the geologically characteristic function, the results of the deconvolution process are only as accurate as the function expressing the source pulse. As discussed by A. Ziolkowski in an article entitled Why Don't We Measure Seismic Signatures? (Geophysics, Vol. 56, No. 2, Feb. 1991, pp. 190-201), which is hereby incorporated by reference, several problems are associated with source pulse deconvolution in the prior art. In general, deconvolution methods may be separated into statistical and deterministic methods. Statistical deconvolution methods commonly estimate a wavelet from the seismic data. The wavelet is a model seismic pulse, usually composed of one or two cycles. The estimated wavelet is then applied to the deconvolution process to derive the geologically characteristic function. Although the methods of estimating a wavelet are numerous, none bases the wavelet estimate on actual measurement of the source pulse waveform. Instead, the wavelet function depends on assumptions and guesses relied upon by the analyst to estimate the wavelet from the reflected pulse data. Consequently, determination of the wavelet depends not upon an objective test, but the subjective judgment of the interpreter of the data. As indicated by Ziolkowski (pp. 193-95), estimating the wavelet using statistical methods relies on a combination of assumptions about the properties of the source pulse and the geology that bear little or no theoretical justification.
Because the source pulse function derived from the estimated wavelet is inaccurate, the deconvolution process cannot determine an accurate impulse response function for the geological formation. When the reflected pulse function is deconvolved using the deflective source pulse function, the result is an erroneous impulse response function of only marginal accuracy. Conclusions drawn from the resulting function regarding the underground formation are at best imperfect, and costly as well.
Because of the shortcomings of the statistical data analysis methods, deterministic methods are generally recognized as superior methods of analyzing seismic data. Deterministic methods involve directly measuring the waveform of the source pulse. Thus, when the reflected pulse data is received, the known source pulse function may be effectively deconvolved from the data to determine the geologically characteristic function.
In the marine environment, the source pulse function may be determined using a hydrophone towed directly beneath the source. When the source emits an acoustic pulse, the hydrophone records the waveform of the pulse as it propagates through the water. The analyst then uses the measured waveform in the deconvolution process to separate the source pulse from the reflected wave and determine the geologically characteristic function. Generally, the waveform recorded by the hydrophone is directly applied to the convolution process without substantive alteration.
Although this method uses a source pulse function that is based on the actual pulse instead of an estimated wavelet, the source pulse function generated by the measuring hydrophone is not an accurate expression of the source pulse generating the reflected waves. The waveform received by the measuring hydrophone is not identical to the waveform received at the distant bottom. In deep water, the far field waveform for the pulse approximates the waveform received at the bottom. The hydrophone, however, is much closer to the source array than the floor, so that the hydrophone does not record the far field waveform. The disparity exists because the depth of the source is sufficiently significant in relation to the distance of the hydrophone from the source that the waveform measured by the hydrophone differs from the far field waveform. The difference of distances generates a difference of phase spectra for the two waveforms. Therefore, when the measured source pulse waveform is applied to the deconvolution process for the reflected pulse, the deconvolution process applies a waveform that does not accurately correspond to the waveform that generated the reflected pulses. Consequently, any results derived from the reflected pulses are likely to be imprecise and inaccurate.
The disparity between the phase spectra of the measured pulse and the far field pulse is proportional to the difference between the distance from the source to the measuring hydrophone and from the source to the marine floor. Consequently, the further the hydrophone is placed from the source, the more accurately the hydrophone measures the waveform actually incident upon the marine floor. Unfortunately, technical and practical considerations limit the distance the hydrophone may be placed from the source. Thus, the phase spectrum measured at the hydrophone always includes an error which degrades the data.